The catalytic reaction cycles

As an introductory example we take one of the key reactions in cleaning automotive exhaust, the catalytic oxidation of CO on the surface of noble metals such as platinum, palladium and rhodium. To describe the process, we will assume that the metal surface consists of active sites, denoted as We define them properly later on. The catalytic reaction cycle begins with the adsorption of CO and O2 on the surface of platinum, whereby the O2 molecule dissociates into two O atoms (X indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site ) ... 

Bis(p -octadienediyl-Ni11 species are shown (i) to be thermodynamically highly unfavorable, thus indicating them to be sparsely populated, and (ii) not to be involved as reactive intermediates along any viable path either for allylic isomerization or for reductive elimination. This leads to the conclusion, that bis(p ) species play no role within the catalytic reaction cycle. 

Figure 2. The catalytic reaction cycle of the Tetrahymena ribozyme (E), showing the binding and docking reactions (leading to the formation of E-S complex), followed by a bond cleavage breaking step (the rate constant for which is kchem) and release of the 5 -fragment in the multiturnover steps (rate constant equals k t).

CATALYSIS. Any condition promoting formation will tend to speed up the reaction rate, and catalysts are thought to accomplish rate enhancement chiefly by stabilizing the transition state. Shown in Fig. 8 is an enzyme-catalyzed process in which reactant S (more commonly called substrate in enzymology) combines with enzyme to form an enzyme-substrate complex. This complex leads to formation of the transition state complex EX which may proceed to form enzyme-product complex. The catalytic reaction cycle is then completed by the release of product P, whereupon the uncombined enzyme returns to its original state. 

STEP ONE Using just the different enzyme forms, write out the reaction scheme, such that a closed geometric pattern is generated. In this case (E) occurs at the start and end of the catalytic reaction cycle, and we get ... 

By replacing the natural substrate homoprotocatechuate (3,4-dihydroxyphenyl acetate) with the slower reacting substrate analog 4-nitrocatechol, six intermediates of the catalytic reaction cycle of 2,3-HPCD could be observed experimentally, four of which could even be characterized by X-ray crystal structure analysis [22], Scheme 2.4 gives an overview of the proposed reaction mechanism. 

Because at steady state all the rates that constitute the catalytic reaction cycle are equal, there are several expressions that one can use for fhe rate of CO consumption. We start the derivation with the rate of CO consumption as given by the following expression ... 

FIGURE 6. Schematic representation of the catalytic reaction cycle in flavocytochrome b2. Five redox intermediates of FCB2 during the oxidation of one molecule of lactate at a steady-state turnover rate of 100 sec and the reduction of two molecules of cytochrome c at the rate of 200sec° are shown. Step 4 is the rate limiting step in the steady state and the maximal rates of some of the other electron transfer steps are indicated. Reproduced from Daff et al., 1996 with permission. 

Pa p(CO) = 0.03 Pa p(He) = 1.0 Pa. Open symbols represent the normalized experimental data. The solid lines are obtained by fitting the integrated rate equations of the catalytic reaction cycle (see text) to the experimental data. (C) Five optimized structures (a)-(e) ofAu2C03, with bond lengths in A. The relative stability of these structures is... 

Fig. 1.34. Catalytic CO oxidation by N2O in an ICR mass spectrometer [24]. The ICR signal shown obtained for Fe+ while scanning the double-resonance oscillator to eject ions of a given mass from the ceU. Trace B is obtained with only N2O present in the trap. Trace A results when CO is added in addition to N2O to the ion trap. The increase in Fe+ signal after adding CO is caused by the regeneration of Fe in the catalytic reaction cycle indicated in the inset. This is evidenced by the double resonance at 72amu (FeO ), which indicates that FeO+ is reacting to Fe ...

Fig. 1.64. Production concentrations as a function of the reaction time for three different reaction temperatures and different reactant gas concentrations, (a) T = 300 K p(02) = 0.12Pa p(CO) = 0.12 Pa p(He) = 1.2Pa. (b) T = 150K p(02) = 0.04 Pa p(CO) = 0.04 Pa p(He) = 1.0 Pa. (c) T = 100K (Oa) = 0.02 Pa p(CO) = 0.03 Pa p(He) = 1.0 Pa. Open symbols represent the normalized experimental data (squares), Aua (circles), AuaOa (triangles), Au2(C0)02 ). The solid lines are obtained by fitting the integrated rate equations of the catalytic reaction cycle (1.65) to the experimental data...

It has been explained in a previous section that diffusion of hexane and isohexane can be considered to be fast. From a combination of theoretical and experimental data, a reaction energy scheme corresponding to the catalytic reaction cycle that converts zz-hexene into z-hexene has been deduced. This is shown in Fig. 17 [136, 137]. As in Section V, adsorption on the siliceous part of the zeolite micropore is considered to be independent of proton activation. 

Regulation processes are integral constituents of the catalytic reaction cycle. 

The local surface concentration of molecules and adatoms can change over the course of a catalytic reaction. This is intrinsic to the catalytic reaction cycle. Reconstruction can... 

Figure 3.11. The catalytic reaction cycles for the total oxidation of (a) CO and (b) CH4 on oxide catalysts.

Towards the end of the CIAP programme some researchers had turned their interest to the potential input of reactive chlorine radicals on stratospheric ozone. In the most thorough of these studies, Stolarski and Cicerone [50] calculated substantial ozone depletions if inorganic chlorine were present in the stratosphere at a volume of mixing ratio of 1 nmol/mol of air. Odd oxygen destruction would take place by the catalytic reaction cycle (21) + (22). This reaction sequence is very similar to the... 

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